The invention relates to a method for reducing the cutting impact in a precision blanking press equipped with a hydraulic main drive, a main piston in the press making a stroke movement between UT and OT during rapid traverse mode and performing the cutting or shaping operation in the power stroke, the main piston being guided in a main cylinder chamber of the base and supporting a table top, wherein the pressure chambers of the main piston are acted upon by the working pressure of a hydraulic fluid of a hydraulic system, the pressure being predetermined by a central control system and generated by a hydraulic pump unit.
The invention further relates to an apparatus for carrying out the method, comprising a main cylinder chamber disposed in the base of a hydraulically-driven precision blanking press, the main piston/ram being guided in the chamber and which can be acted upon by hydraulic fluid by way of pressure chambers, making a lifting motion in rapid traverse mode between UT and OT in the direction of the stroke axis and supporting a table top, and a hydraulic system for supplying the pressure chambers with the hydraulic fluid, which is adjustable to a predetermined working pressure by way of a central control system, the hydraulic system comprising at least one hydraulic pump unit.
Cutting impact on presses and the causes thereof have been known for a long time. The cutting process is known to consist of a clamping step in which the sheet material to be cut is clamped between a die block and a hold down device (guide plate), whereupon the cutting punch is placed onto the sheet material, a pretensioning step in which the cutting system, consisting of the sheet material, the blanking die and the press, is pretensioned elastically until the flow boundary is reached, a plastic deformation step in which the sheet material is cold-hardened to a maximum value as the cutting force increases, and upon reaching the limit of deformability the sheet material breaks through with the formation of cracks, a separation step in which the cut part is separated from the sheet material, the cutting force decreasing suddenly and the potential energy stored in the system being released, allowing the die and the press to suddenly release in the form of cutting impact, and release of elastic stresses, which cause the material near the cut surface to spring back and cause the punch and the die block to wear out.
There are a number of solutions to this which deal with the damping of the cutting impact on presses. DE 22 48 024C, DE 23 50 378 C2, DE 26 21 726C, DE 26 53 714 C, DE 102 52 625 B4, DE 103 39 004 B4, DE 10 2005 053 350 A1 and DE 10 2009 39 004 B4 disclose cylinder-piston units, with or without sensors, which act separately from the ram and which are used to damp the reverse deformation of the press frame. Other known solutions (DE 25 12 822 C, DE 10 2009 39 004 B4) try to absorb the cutting impact using an elastic press frame length.
The prior art according to DE 28 24 176 C2 discloses a hydraulic press having a large stroke for impact-free cutting or stamping, with a path-dependent servo-hydraulic control system for the ram movement, wherein a working piston for the press ram and at least one counterforce piston can be simultaneously acted upon with pressure fluid, wherein a set point transmitter controls the piston movements by way of a servo-valve for specifying a ram movement profile, the transmitter being an element of a control circuit, and wherein the working cylinder chamber for driving the press ram and the counterforce cylinder are spatially separated from one another.
DE 43 08 344 A1 discloses a method for controlling the drive of a hydraulic press for forming and/or cutting sheets or the like, the press comprising a piston-cylinder unit that can be acted upon on two sides for driving a press ram, wherein the drive piston is acted upon by a hydraulic medium in the manner of path displacement. What is provided is a hydraulic storage unit having the maximum working pressure, the unit supplying the piston of the piston-cylinder unit with hydraulic medium of the same pressure on both sides.
DE 10 2007 027 603 A1 describes a hydraulic drive, in particular for a press, a stamping or nibbling machine, and a method for controlling a hydraulic drive. The known drive has a dual-acting working cylinder with a piston which comprises a first working surface acting in the inward direction and a second working surface acting in the outward direction, each of the surfaces confining a pressure chamber, wherein at least two different pressures can be enacted by way of an actuator so as to retract and extend the piston into and out of the working cylinder toward at least one of the working surfaces. The hydraulic drive has an adjusting unit which enables a throttled reflux of hydraulic medium out of the first pressure chamber confined by the first working surface, wherein the drive comprises a control means for controlling the adjusting unit in such a manner, during or prior to a sudden acceleration of the piston due to the sudden release of force thereof, as to effect a hydraulic damping of the movement of the piston by way of a direct or indirect throttling of the reflux of hydraulic medium out of the first pressure chamber.
DE 10 2006 039 463 A1 further proposes to reduce the cutting impact by providing, within a closed hydraulic system, two different opposing forces acting on the ram during a cutting step, wherein the forces are switched depending on the path of the ram.
All of these known solutions have the common disadvantage of needing to use additional hydraulic-mechanically acting means to counteract the cutting impact; these additional means significantly increase the complexity of the press while at the same time complicating the design and increasing costs. In particular, additional adjusting and throttling units placed between the pressure chambers of the drive piston, or separate closed hydraulic systems, increase the reaction time and thereby the sluggishness of the overall system. Therefore, these solutions have not come to fruition. Moreover, throttling units have the disadvantage that they heat up, causing irreversible energy losses.